In general, when etching a silicon single crystal substrate with a chemical liquid, there are two methods, i.e., a method of etching with an acid-based etching liquid which is a mixed aqueous solution obtained by adding components such as hydrofluoric acid and nitric acid, and a method of etching with an alkali-based etching liquid which is an aqueous solution of potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAH) or the like.
When using the acid-based etching liquid, the silicon surface is oxidized by a component having oxidizing properties such as nitric acid to generate silicon oxide, and this silicon oxide is dissolved by hydrofluoric acid or the like in the form of silicon fluoride, and thus etching is progressed. Etching with the acid-based etching liquid is characterized in that etching is isotropically progressed. The etching rate of the acid-based etching liquid varies depending on the mixing ratio between hydrofluoric acid and nitric acid and is about 1 to 100 μm/min. However, since the acid-based etching liquid corrodes metal wiring of copper, aluminium or the like, it is difficult to use the liquid in a process in which a metal coexists.
Meanwhile, when using the alkali-based etching liquid, silicon is dissolved by hydroxy anion in the liquid in the form of silicate ion, and at this time, water is reduced to generate hydrogen. Unlike the acid-based etching liquid, when etching with the alkali-based etching liquid, etching of single crystal silicon is anisotropically progressed. This is based on the difference of silicon dissolution rate among crystal face orientations of silicon, and is called “crystal anisotropic etching”. Even in the case of polycrystal, microscopically, etching is progressed with anisotropy being maintained, but since face orientations of crystal grains are randomly distributed, macroscopically, it appears that isotropic etching is progressed. In the case of amorphous, isotropic etching is progressed both microscopically and macroscopically.
As alkaline etching liquids, in addition to aqueous solutions of KOH and TMAH, aqueous solutions of sodium hydroxide (NaOH), ammonia, hydrazine, etc. are also used. In the process of etching a substrate of single crystal silicon using these aqueous solutions, a long period of processing time in the range of from several hours to several ten hours is often required though it depends on a desired processing shape, temperature conditions for the treatment, etc.
For the purpose of reducing the above-described processing time, chemical liquids showing a high etching rate have been developed. For example, Patent Document 1 discloses a technique of using an aqueous solution obtained by adding hydroxylamines to TMAH as an etching liquid.
Patent Document 2 discloses a technique of using an aqueous solution obtained by adding a specific compound such as iron, iron (III) chloride and iron (II) hydroxide to TMAH as an etching liquid. The document also discloses that a particularly preferable etching rate is obtained by combined use of iron and hydroxylamine.
Patent Document 3 discloses a technique of using an aqueous solution obtained by adding hydroxylamines to KOH as an etching liquid.
Patent Document 4 discloses an etching liquid consisting of an alkaline reducing compound and an anticorrosive (sugar, sugar alcohol, catechol, etc.).
Patent Document 5 discloses a technique of suppressing decomposition of hydroxylamine by means of addition of an acid to an alkali.
Patent Document 6 discloses a technique of suppressing decomposition of hydroxylamine by means of addition of an alkali salt to an alkali and hydroxylamine.
Patent Document 7 is mentioned as a patent comprising KOH, hydroxylamine and urea, but the patent relates to the development of a photoresist, and the document does not describe any silicon etching liquid or silicon etching method. In addition, the document does not describe any cyclic compound having a specific structure, which has a thiourea group and wherein N and N′ are linked.
Patent Document 8 discloses a silicon etching liquid, which anisotropically dissolves single crystal silicon, and which is an alkaline aqueous solution containing (1) at least one alkaline hydroxide selected from potassium hydroxide, sodium hydroxide and tetramethylammonium hydroxide, (2) a hydroxylamine and (3) a thiourea group. However, the document does not describe any cyclic compound having a specific structure, which has a thiourea group and wherein N and N′ are linked.